MatHeMats - Distribution functions for the description of heterogeneous metallic microstructures
Final Report Abstract
Advanced high-strength steels (AHSS) are extensively used in many industrial applications. Among the various AHSS, dual-phase (DP) and complex-phase (CP) steels are most extensively used due to their favorable combination of strength and ductility. CP steels exhibit superior local formability while DP steels show better global formability. In this project, both experimental and numerical investigations are carried out to establish the correlation between the heterogenous microstructures and the resulting mechanical properties with special consideration of local microstructural gradients. The correlation among the compositional, microstructural and property heterogeneities is explored by employing four commercials grade DP and CP steels. A correlative characterization approach combining various electron microscopic techniques and nanoindentation is developed and utilized to characterize the heterogeneities. A novel nanoindentation technique was applied to obtain hardness maps with a largely decreased distance between the adjacent indents (~300 nm), which were successfully overlapped with the microstructural information from electron backscattered diffraction (EBSD) and the elemental distribution from electron probe microanalysis (EPMA). The developed methodology shows great potential in characterizing the increasingly complex and heterogenous microstructure of the AHSS group. As an essential hardening alloy element, Mn alternately enriched and depleted in bands parallel to the rolling direction, whereas the local C content is closely associated with spatial distribution of individual phases. Despite their identical cast chemistry, the results of DP800 and CP800 indicate a combined effect of intercritical annealing temperature and local chemistry, especially the Mn bands, on the phase transformation behavior and the final microstructure. The Mn segregation bands and their impact on martensite morphology and location in these two steels are reflected by the local hardness distribution. As a measure of local formability, hole expansion tests were conducted together with SEM investigations to study the influence of edge condition and microstructure on the damage initiation and evolution in DP and CP steels. Obtained high-resolution hardness maps are used to describe the local property heterogeneities and deliver large data sets for statistical evaluations. The Shannon entropy, which originated from the information theory, is adopted in this study to quantify the heterogeneity degree of hardness distribution. The obtained Shannon entropy of hardness distributions shows a good alignment with the hole expansion ratio of investigated materials. Besides, a mathematic approach is developed to quantitatively describe the microstructural and hardness heterogeneity, based on which representative volume element (RVE) models are established to include the influence of microstructural heterogeneity on materials’ deformation behavior. Gradients of properties were proposed as descriptors of the microstructure, which allow estimating resistance to local fracture. The calculation of property gradient based on the sensitivity analysis methods was proposed. This methodology allows calculating gradients for the data given mode, which is independent from the source of the data. Gradients of properties were calculated for CP and DP microstructure using RVE and statistically similar representative volume elements (SSRVE) representations of the microstructure. Additionally, gradients were calculated using the high-resolution hardness maps. The measures based on the gradients of properties are proved effective to quantify the microstructural heterogeneity and can be further developed to predict the failure of material during the forming process. In summary, the present work deepens and expands the current understanding and knowledge of the heterogeneous microstructure and, hopefully, the developed approaches and derived conclusions provide a guideline for future material design by fabricating the heterogeneities of microstructure to achieve more balanced properties.
Publications
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Correlation between microstructure and micromechanical properties of complex phase steels, KomPlasTech 2019, 13-16 Jan. 2019, Zakopane, Poland
Chang, Y., Li, X. Haase, C., and Bleck, W.
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Evaluation of using distribution functions for mean field modelling of multiphase steels, Procedia Manufacturing, 27, 2019, 72-77
Szeliga, D., Chang, Y., Bleck, W., Pietrzyk, M.
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Fast model for phase transformations during cooling of pre-annealed multiphase steels, Computer Methods in Materials Science, 19, 2019, 150-161
Szeliga, D., Kuziak, R., Zalecki, W., Pidvysots’kyy, V., Chang, Y., Bleck, W., Bachniak, D., Pietrzyk, M.
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Investigation of microstructural gradient of complexphase steels through experiment and modeling, Euromat 2019, 1-5 Sep. 2019, Stockholm, Sweden
Chang, Y., Szeliga, D., Pietrzyk, M. and Bleck, W.
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Investigation of microstructuremechanical property correlation of complex-phase steels by electron microscopy and nanoindentation. Metal Forming 2020,13-16 Sep. 2020, E-conference, Krakow, Poland
Chang, Y., Hangen, U., Szeliga, D., Pietrzyk, M., and Bleck, W.
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(2021) Compositional heterogeneity in multiphase steels: characterization and influence on local properties. Materials Science and Engineering A 827 142078
Chang, Y., Haase, C., Szeliga, D., Madej, L., Hangen, U., Pietrzyk, M., and Bleck, W.
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(2021). Criterion for microcrack resistance of multi-phase steels based on property gradient maps. CIRP Annals, 70(1), 243–246
Madej, L., Chang, Y., Szeliga, D., Bleck, W., and Pietrzyk, M.
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(2021). Revealing the relation between microstructural heterogeneities and local mechanical properties of complex-phase steel by correlative electron microscopy and nanoindentation characterization. Materials & Design, 203, 109620
Chang, Y., Lin, M., Hangen, U., Richter, S., Haase, C., and Bleck, W.
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Correlation between microstructural heterogeneity and stretch-flangeability of dual-phase and complex-phase steels. KomPlasTech 2021, 8-9 Mar. 2021, online event, Krakow, Poland
Chang, Y., Lin, M., Lian, J., Haase, C., and Bleck, W.
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Quantification and correlation of the microstructural heterogeneity and stretch-flangeability of high-strength dual-phase and complex-phase steels, 40th IDDRG Conference 21 Jun. - 2 Jul. 2021, online event, Stuttgart, Germany
Chang, Y., Lin, M., Lian, J., Hangen, U., and Bleck, W.
